It is common practice with four wheel drive type vehicles to use a central gear box or differentials, the output of which is fed directly to the individual driven wheels of a vehicle. In applications where high ground clearance is required it is desirable to have the axle shaft of each wheel couple to a position higher than the wheel spindle using a geared hub. The axle shaft connects to a pinion gear of the geared hub that is meshed with an output gear normally splined to the wheel spindle. The use of the two gears at the wheel allows for increased ground clearance and a reduction in the gearing in the central gear box because some of this gearing is now taking place in the geared hub.
The geared hub design is commonly used by AM General LLC in its HMMWV military product and in the civilian Hummer vehicles. An example of a prior art geared hub is provided as FIG. 1. A geared hub is typically used with a portal axle where the axle tube is above the center of the wheel hub. Using the portal axle provides increased ground clearance, as both the axle tube and differential casing are tucked up higher under the vehicle. Geared hubs can also be used with inverted portal axles where the axle tube is below the center of the wheel hub that can be used, for example, in a low floor bus that provides easier passenger access.
Typical braking mechanisms for vehicles use a friction-based brake that applies a fixed surface against a rotating drum or disk that is coupled to the wheel of the vehicle. The friction material applied against a drum or disc resists the rotation of the wheel and stops the vehicle due to the friction force between the tire and the road surface. The kinetic energy of the vehicle is converted to heat energy by the friction surfaces.
Friction-based brakes are subject to wear of the friction surfaces and can generate large amounts of heat that both lead to maintenance and performance issues. The friction surfaces, such as drums and discs, wear down and require frequent replacement, and can be a large expense when managing a large fleet of vehicles. The rate of wear can be increased due to temperature, vehicle load, foreign debris between the friction surfaces. For example, in a loose soil environment the heat and soil/sand can increase wear on the friction surfaces and other braking components requiring premature replacement. A buildup of heat on the friction surfaces from heavy or sustained braking can result in a loss of stopping power due to brake fade. Friction brakes are also subject to brake fade from overheating fluid, or debris between the braking surfaces.